Rails4.2でmysql2を認識しない mysql2 is not loaded?

**追記: Rails4.2.5以降では以下の問題は解消しています**

## Error log: but the gem is not loaded???

な、なぜ...

```
/Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_adapters/connection_specification.rb:177:in `rescue in spec': Specified 'mysql2' for database adapter, but the gem is not loaded. Add `gem 'mysql2'` to yo
ur Gemfile (and ensure its version is at the minimum required by ActiveRecord). (Gem::LoadError)
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_adapters/connection_specification.rb:174:in `spec'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/connection_handling.rb:50:in `establish_connection'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/railtie.rb:120:in `block (2 levels) in <class:Railtie>'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:38:in `instance_eval'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:38:in `execute_hook'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:45:in `block in run_load_hooks'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:44:in `each'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activesupport-4.2.4/lib/active_support/lazy_load_hooks.rb:44:in `run_load_hooks'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/base.rb:315:in `<module:ActiveRecord>'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/activerecord-4.2.4/lib/active_record/base.rb:26:in `<top (required)>'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/spring-1.4.0/lib/spring/application.rb:319:in `active_record_configured?'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/spring-1.4.0/lib/spring/application.rb:251:in `disconnect_database'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/spring-1.4.0/lib/spring/application.rb:97:in `preload'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/spring-1.4.0/lib/spring/application.rb:143:in `serve'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/spring-1.4.0/lib/spring/application.rb:131:in `block in run'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/spring-1.4.0/lib/spring/application.rb:125:in `loop'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/spring-1.4.0/lib/spring/application.rb:125:in `run'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/spring-1.4.0/lib/spring/application/boot.rb:18:in `<top (required)>'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
        from /Users/user/.rbenv/versions/2.2.3/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
        from -e:1:in `<main>'
```

もちろんmysql2を読み込んでいる

```rb
# $ head Gemfile
source 'https://rubygems.org'


# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.2.4'
# Use mysql as the database for Active Record
gem 'mysql2'
```

## How to fix it

** Rails4.2.5では下記のissueは対応済みなので最新を利用してください。 **

- https://github.com/rails/rails/issues/21544
 - Rails(mysql2 adapter)の方で0.3系依存で定義してしまっているので、mysql2最新の0.4.0を受け付けない
 - Gemfileで`gem 'mysql2', '~> 0.3.20'`のように0.3系に設定すれば問題ない

```diff
diff --git a/Gemfile b/Gemfile
index 4255ffd..2941d8f 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,7 +4,7 @@ source 'https://rubygems.org'
 # Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
 gem 'rails', '4.2.4'
 # Use mysql as the database for Active Record
-gem 'mysql2'
+gem 'mysql2', '~> 0.3.20'
 # Use SCSS for stylesheets
 gem 'sass-rails', '~> 5.0'
 # Use Uglifier as compressor for JavaScript assets
diff --git a/Gemfile.lock b/Gemfile.lock
index 36ac5c1..442120d 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -70,7 +70,7 @@ GEM
     mini_portile (0.6.2)
     minitest (5.8.0)
     multi_json (1.11.2)
-    mysql2 (0.4.0)
+    mysql2 (0.3.20)
     nokogiri (1.6.6.2)
       mini_portile (~> 0.6.0)
     rack (1.6.4)
@@ -143,7 +143,7 @@ DEPENDENCIES
   coffee-rails (~> 4.1.0)
   jbuilder (~> 2.0)
   jquery-rails
-  mysql2
+  mysql2 (~> 0.3.20)
   rails (= 4.2.4)
   sass-rails (~> 5.0)
   sdoc (~> 0.4.0)
```